Pressurised metered dose inhaler canisters are filled with a mixture of propellant and active ingredient. Legislation dictates a maximum leak rate of the combined propellant/ingredient, typically measured in mg/day or g/year, for each product. Canisters found to be leaking at a rate greater than this should be eliminated from production batches upon detection. The specific leak rate is subject to the product type/canister size but is of the order of 1 mg/day.
The most common method of determining whether the can is leaking is to weigh the can twice, with a sufficient period of time in between measurements to accurately assess if the mass of propellant lost in the elapsed time corresponds to a leak rate above the specification. In practice, the accuracy of a typical online check weigh device is <0.05 g and the tolerance is set to reject cans falling more than +/−0.5 g outside of the target fill weight. In the four week dwell period between tests, a typical check weigh device could expect to catch no more than 50% of cans leaking at a rate of 15 mg/day, which is more than 10 times higher than the legislative requirements. In principle, cans leaking as much as 30 mg/day are able to enter the consumer market.
To mitigate the possibility of a systemic production issue generating leaks between 1-30 mg/day that might otherwise escape detection, a sample of cans is extracted from the production line for laboratory analysis with more precise instrumentation. In this instance, the period between weights is typically three days. This type of analysis is capable of detecting leaks at specification. However, the testing is only carried out on a small fraction of overall production and the cans are not returned to production.
Other methods of online leak detection lack either the speed or sensitivity to achieve the specification requirements. These other technologies may include optical absorption techniques, flame ionisation detection, photo-ionisation detection, pressure loss/decay, O2 sensors or other oxygen displacement techniques, conductivity, submersion, acoustic or other water bath based systems. In the event of a production or component fault resulting in leaking canisters, the earliest that small leaks are likely to be detected is three days after production and the longest period may be up to one month. At a typical rate of 200,000-300,000 canisters per day, this represents a sizeable volume of unfit goods that may accumulate in the intervening period before a leak or process issue is identified.